This invention relates generally to heaters and more particularly to an electric cast-metal heater especially adapted for heating plastic extrusion equipment.
Electric heaters are typically used to heat the barrel of an extruder to initiate the extrusion process. Thereafter the heater becomes less necessary, at least in instances where the friction generated by the movement of material through the barrel provides sufficient heat to maintain the material in a fluid state. In those instances, the heat generated by such movement may become so great that it must be dissipated to avoid adverse effects on the extruded product. In large systems generating a correspondingly large amount of heat, cast-aluminum heaters with integral (cast-in) electrical heating elements and cooling tubes are frequently used. An alternate approach is the use of a finned cast-aluminum heater and high-capacity blower system for cooling the heater. On smaller machines the use of a plurality of relative narrow electric heater bands is common. Such bands are usually secured (e.g., clamped) to the barrel at spaced intervals and air then blown over the heaters and exposed portions of the barrel to provide the necessary cooling.
All of the heating/cooling systems described above have serious drawbacks. Thus, the cast-aluminum heaters with integral cooling tubes or fins are expensive, and the separate band heaters require individual mounting and numerous electrical connections (two per heater) making handling and installation inconvenient. Moreover, use of separate band heaters has resulted in barrel warpage. Accordingly, there is a need in the industry for an improved heater which avoids these drawbacks.